Axial flow turbo-machine blade with abrasive tip



w. B. MOYER Aug. 10, 1965 AXIAL FLOW TURBO-MACHINE BLADE WITH ABRASIVE TIP Filed May 4, 1964 FIG.2

ROTATION INVENTOR WAYNE B. MOYER, BY M 'W HIS ATTORNEY.

FIG.4

l2\ HO saw United States Patent 3,19%,336 AXIAL FLQW HiliiEQ h ZAQHENE BLADE WEEK-i ABRASi' /E Tl? Wayne E. Meyer, Schenectady, N.Y., assignor to General Electric Company, a corporation of New York Filed May 4, 1964, Ser. No. 364,591 8 Claims. (Q1. 253-77) This invention relates to axial flow turbo-machine blades, particularly to high-temperature gas turbine blades whic have no integral shrouds but are closely surrounded by a stationary shroud member. It relates particularly to a special blade construction having a special blade fastening structure and adapted to safely removing any interfering portions of the surrounding stationary shroud member without damage to the other blades.

Although the invention may be applicable to shroudless blade members of both axial flow compressors and turbines, it has particular utility in shroudless high-temperature turbine blades. From an aerodynamic standpoint, the turbine designer wants to have the absolute minimum radial clearance between such shroudless blades and the surrounding stationary shroud ring. Such shrouds are often made in two or more segments, suitably supported relative to the rotor member. In spite of the designers best efforts, there is a chance that, under the influence of transient temperature conditions, the shroud member will go out of round, or the rotor and particularly the slender blades thereof will as the temperature increases grow radially faster than the comparatively heavy surrounding shroud ring will increase in diameter. There is also the possibility that the rotor axis may shift slightly eccentric relative to the shroud ring, due to misalignment of the hearings or the small degree of eccentricity permitted by the necessary annular clearance space in the hearing, or possibly due to difierential thermal expansion of the support members positioning the bearing relative to the shroud ring. All these effects tend on occasion to cause interference between the rotating blades and the sur rounding shroud ring. Therefore, as a practical matter, the turbine designer hitherto had to provide a radial clearance between bucket tips and shrould ring substantially greater than is desirable from an aerodynamic standpoint. The problem is to reconcile to the greatest extent possible the smallest practicable radial clearance space between blade tips and shroud which still prevent mechanical interference therebetween under all operating conditions.

Accordingly, the primary purpose of the present invention is to provide an improved axial flow turbo machine blade structure and arrangement wherein only one blade in each circumferential row of blades is slightly longer than the remaining blades in that row and is provided with a special tip coating of abrasive material which will automatically remove sufficient material from any interfering portions of the stationary shroud ring, so as to insure clearance between the shroud ring and remaining buckets.

Another object is to provide a turbo-machine blade structure of the type described requiring a minimum amount of the relatively expensive high-temperature-resisting abrasive material.

A further object is to provide an axial fiow turbomachine blade structure permitting the designer the widest possible choice of materials for the blades and the surrounding stationary shroud ring.

A still further object is to provide axial flow turbomach-ine blade structure in which all the blades in a given row are identical, with the single exception of a special blade which is assembled last in the rotor (and disassembled first), this special bucket incorporating an abra- Bibdfidh Patented Aug. 1%, 1965 sive or cuttin tip portion disposed to prevent interference between the remaining blades and the shroud.

The general object of the invention is to provide an axial fiow turbo-machine blade and shroud arrangement which permits the absolute minimum required radial clearance space between blade tips and shroud, since any interference automatically effects removal of material rom the interfering portion of the stationary shroud, with no material removed from the tips of the other buckets and only a minimum of material, if any, removed from the special abrasive-tipped blade.

Perhaps the most obvious way to accommodate for possible interference between the tips of shroudless axial flow turbo-machine blades and the cooperating stationary shroud member is to provide each blades with a thin tip portion which is readily worn away if any contact occurs with the shroud. The next most obvious expedient is to provide the stationary shroud member with reduced section portions which will readily wear away without substantial damage to the blade tips. These expedients are disclosed in the US. Patent to Parson et a1. 899,319, issued September 22, 1908. It has also been previously suggested that an abrasive or cutting member may be disposed on the stationary shroud ring, so that an appropriate amount of the rotor blade tips will be automatically removed in the event of interefrence between the blades and shroud. This is disclosed in the US. Patent to Zimmerman2,507,079, issued May 9, 1950. It can be shown, however, that removing material from the tips of the blades creates a larger aggregate annular clearance space between rotor and stator than if the blade tips remain unaffected and the interfering material is removed from the stationary shrould structure. Such an arrangement is disclosed in US. Patent to Brandt, Jr., et al.- 2,840,343, issued June 24, 1958. With such an arrange ment, it is necessary to provide special shroud material which can be removed efiectively by the blade tips with out substantial damage to the blades. The Brandt et a1. patent gives one example of such shroud segments with special wear-away material, intended to be removed by the blade tips when interference occurs. Other expedients for providing easily worn-away material for the shroud rings have been proposed, for instance the honeycomb shroud structure of the US. Patent to Daunt et al. -3,053,694, issued Sept. 11, 1962, in which the interstices of the honeycomb structure are filled with a special readily abradab.e material. Such special or composite shroud structures are inherently expensive and constitute a source of service difficulties, due to breakdown of the composite shroud under the high-temperature conditions occurring in a gas-turbine power plant over a long period of time.

Accordin ly, an important general object of the present invention is to provide a special abrasive-tipped bucket which will safely remove interfering material from the surroundng shroud ring member, while giving the turbine designer the widest possible choice of materials for the blade and shroud, and without resorting to highly special or composite shroud structures.

Other objects and advantages will become apparent from the following description, taken in connection with the accompanying drawings in which:

FIG. 1 is an elevation view, partly in section, showin a number of adjacent turbo-machine blades including a single special abrasive-tipped bucket in accordance with the invention,

FIG. 2 is an end view looking down on the group of buckets of FIG. 1, and

FIG. 3, taken on the plane 3-3 in FIG. 2, is an en larged detail section of the abrasive tip of the special locking bucket.

FIG. 4 is a sectional detail taken on the line 44 of FIG. 2.

Generally stated, the invention is practiced by providing a single special bucket in each circumferential row of turbo-machine blades, this single non-standard bucket having a special dove-tail base permitting it to be assembled last (and disassembled first, when the buckets are to be removed from the wheel), and having also a special abrasive tip portion projecting radially beyond the common tip radius of the remaining standard buckets, so that in the event of interference with the stationary shroud ring, the abrasivetipped bucket will remove portions of the shroud material from the interfering portions thereof, to prevent any contact between the shroud and the remaining standard buckets.

Referring now more particularly to FIG. 1 of the drawing, the invention is shown as applied to a circumferential row of shroudless buckets, only three of which are shown in the drawing. The special abrasive-tipped bucket is indicated generally at 1, and it will be seen that this special blade is significantly longer than'the adjacent standard blades 2, 3. It is to be noted that all the remaining buckets on the wheel are identical to the blades 2, 3.

In addition to having the special abrasive-tipped portion 4, the bucket It has a special or non-standard dovetail base especially arranged to permit the bucket 1 to be assembled last after all the remaining buckets 2, 3, etc., have been assembled to the wheel, the bucket 1 also being the first one to be removed in the event some or all of the blades are to be removed for repair or replacement.

Referring first to the dovetail base arrangement for securing the blades to the wheel, it will be seen from FIG. 1 that the wheel rim portion 5 is provided with axially-extending dovetail grooves 6 having axially extending serrations cooperating with mating serrations on the dovetail bases of the buckets 1, 2, 3, respectively.

The small sectional portion cut away from the base of the bucket 2, taken in connection with the end view of the buckets shown in FIG. 2, will serve to illustrate the method of locking the buckets against axial displacement in the dovetail grooves 6. This means comprises a small key member 7, disposed in cooperating recesses formed in the root platform portion 3 of the bucket 2, and partly in the radially outermost portion of the dovetail tooth 9 formed between adjacent dovetail grooves 6. The method of operation of this key device will be seen from the following brief description of the method of assembling the buckets.

The bucket member 2 is the first one to be inserted axially into its groove 6. The small key member '7 is then inserted in a circumferential direction into the cooperating recesses in the blade platform 8 and the dovetail-tooth 9. The next adjacent bucket (not shown) is then inserted in its dovetail groove, this adjacent blade then serving to close the recess occupied by the key so that it cannot become accidentally displaced. It will be apparent that as each bucket is inserted into its dovetail groove 6, it serves to retain in place the previouslyassembled key member? This assembly sequence is used for all the buckets progressing counter-clockwise from the first bucket 2 to the next-to-last bucket 3. It

then becomes necessary to insert the specially designed bucket 1. The dovetail base la of this bucket is shaped generally like the others, including a small recess in the platform portion thereof, indicated at 11A in FIG. 1. It is obviously impossible here to use a keying device such as 7, and in lieu thereof the dovetail tooth to is provided with an axial groove 11, a cross-section of which is shown in FIG. 1 and the plan shape of which is shown in FIG. 2. It will be apparent that this groove 11 opens through the side surface of the dovetail toot 19. Thus, a cylindrical key member, shown as a short edges are for the most part disposed at an cylindrical dowel 12, can be inserted axially in groove 11 and then caused to move radially outward so that the outer end of the dowel engages the recess Ella in the blade platform 8a. The key 12 is maintained in this radially outer position by an axially disposed pin 13 which is driven into the groove 11 in the clearance space defined between the radially inner end of dowel 12 and the inner surface of groove 11. The locking pin 13 may be retained in the groove ll by peening over an adjacent edge of the opening at 13A in the side surface of dovetail tooth Ill Obviously other retaining means could be used for this pin member 11, such as a grub screw threaded into a hole drilled partly in the pin 13 and partly in the tooth 10.

This dovetail base arrangement is particularly advantageous, since the abrasive-tipped bucket is the one most likely to require repair or replacement; and it is, by the nature of the design of the dovetail base used, the bucket which must be removed first in order to disassemble any of the other buckets.

The buckets 1, 2, 3, etc. are surrounded by a shroud ring shown generally at 14 as composed of a number of segmental shroud blocks 14a, 1412, Me, etc. Such segmental stationary shrouds for shroudless gas turbine wheels have become very common in the art, so that the details of the construction of shroud ring 14 need not be gone into in more detail. It needs only to be noted that the radial clearance between the tip of bucket It and the surrounding shroud segments 14 is made just as small as feasible, taking into consideration any out-ofroundness to be expected from the design of the shroud ring and its supporting structure, or any eccentricity to be expected by reason of the characteristics of the bearings supporting the rotor, or any interference to be expected from differential thermal expansion between the shroud and its supporting structure relative to the support means for the bearings.

As indicated above, the bucket 1 is also non-standard by reason of the abrasive tip layer 4- provided at the outer end thereof. The shape and composition of this abrasive layer is shown in more detail in FIG. 3. Here it will be seen that the abrasive layer identified 4 is composed of a matrix of relatively ductile material, having dispersed therein a large number of small sharp-edged abrasive particles 4%, in random orientation, so that any rubbing between the outer surface of the layer 4 and the adjacent surface of the shroud ring will cause a multitude of small cutting edges of the abrasive particles to remove the interfering material from the shroud ring.

It is to be noted further in FIG. 3 that the abrasive tip layer 4 has a cylindrical outer surface 40 coaxial With the axis of the rotor, and also has radial side surfaces 4d, 42, the elements of which are substantially normal to the elements of the cylindrical surface 4c. With this configuration, the bucket tip defines a pair of cutting edges at 4 4g. These square corners, 4;, 4g, provide effective and sturdy cutting edges for the abrasive tip layer 4. it will be obvious from FIG. 2 that these cutting angle to the plane of rotation, by reason of the air-foil shape of the bucket tip. Thus, when the abrassive-tipped bucket 1 encounters an interference condition with the shroud segments 1d, the upstream cutting edge, indicated at 4f in PTGS. 2, 3 will engage the interfering portion of the shroud segments 14 to produce an effective cutting action. It will be noted, however, that the abrasive tip, as shown in FIG. 3, has no rake or clearance angle, the cylindrical outer surface 4c providing a grinding or lapping effect on the interfering portions of the shroud segments 14. That is, the upstream cutting edge 4f provides the initial engagement between the abrasive-tipped bucket and the shroud segments, while the downstream portions of the cylindrical surface 40 produce a grinding or lapping operation on the shroud segments. This arrangement is found to provide a very effective means for safely removing in- U terferring material from the shroud segments with little if any removal of material from the abrasive layer 4.

It will be seen in FIG. 1 that the abrasive-tipped bucket 1 projects radially beyond the adjacent buckets 2, 3, by a distance on the order of .02 inch, this difference being found effective to keep the radial clearance between buckets and shroud to a minimum while insuring removal of interfering portions of the shroud segments by bucket 1 without substantial danger of damage to the remaining standard buckets 2, 3, etc.

The abrasive-tipped bucket having a configuration as shown in the drawings and an abrasive tip portion of the composition described hereinafter, has been found to be so effective in safely removing interfering portions of the shroud ring 14, that it is possible to employ almost any desired materials for the buckets and shroud segments respectively. That is the turbine designer has the widest possible choice of bucket materials and shroud segment materials, depending upon the temperatures to be encountered, and the physical properties of the available material, having due regard to the economies possible by employing the lowest cost material having the desired properties. That is, the buckets may be made of conventional materials used heretofore in gas turbine structures, the same being true of the shroud segments 14, since no limitations are imposed by the employment of the abrasive-tipped bucket i. It is however advantageous to have a shroud material with free machining qualities so that the abrasive tip of bucket 1 will experience minimum difficulties in removing any interfering material from the shroud segments 14. As a practical matter, it is found that an abrasive-tipped bucket as described herein will effectively machine away interfering portions of the shroud segments if the latter are fabricated of any of the usual materials used for such shroud segments in high temperature gas turbines.

By way of example, it may be noted that the buckets 1, 2, 3, etc may be fabricated of any alloy having the following approximate composition:

Percent C 2 Fe Balance The shroud segments 14 may be made of material having the following approximate composition:

A specific example of the composition of the abrasive tip portion 4 and the method of applying it will be seen from the following.

The abrasive tip layer 4 is most conveniently applied by means of an electric welding arc, using filler rod comprised of substantially 30% nickel with tungsten carbide particles dispersed therein. These abrasive particles may be of the order of .002.0l max. dimension, tungsten carbide particles in this form being a commercially available product. It will of course be understood that tungsten carbide in such form has numerous sharp edges and corners which constitute a myraid of small cutting edges when the abrasive layer engages the shroud segments. The nickel of the filler rod provides the relatively ductile matrix 411. The process for applying the abrasive layer may be outlined roughly as follows.

A etallic electrode is used to establish a welding arc, which is shielded by an inert gas, such as argon. The filler rod may be on the order of inch diameter and is progressively fed into the pool of fused metal created by the welding are on the tip of the bucket materials.

It may be noted that, before beginning the Welding process, the bucket is uniformly preheated to a temperature within the range of 400600 F. and is maintained Within this temperature range throughout the welding process, care being taken not to heat the bucket material above 600 F. After establishing the argon-shielded arc, the filler rod is fed into the pool of fused metal created by the arc; and the matrix of nickel containing the dispersed tungsten-carbide particles is readily deposited in a single pass, starting at the thin trailing edge of the bucket and proceeding to the center of the bucket tip, and then starting at the leading edge of the blade and ending again at the center of the bucket. The total build-up of this hard-surfacing material my be on the order of inch, but is subsequently ground to the finished configuration of FIG. 3, so that the radial thickness of the abrasive layer is on the order of inch, on a bucket having a blade 1 about 9" long.

Deposition of the abrasive material may be facilitated by clamping appropriately shaped copper chill blocks to the side surfaces of the blade tip so as to prevent the molten pool created by the welding are from running down the sides of the bucket.

After the abrasive layer is deposited, the bucket is covered with asbestos paper and allowed to cool slowly to room temperature. Immediately after reaching room temperature, the bucket is heat-treated in a furnace with a maximum temperature of 400 F. and then raised to a temperature of 1200 to 1225 P. which temperature is maintained for four hours. Thereafter the furnace is permited to cool to 300 B, after which the buckets are removed and cooled to room temperature in still air.

This process insures that the abrasive layer will be formed without developing cracks therein, and that the resulting layer will be able to withstand the thermal shock conditions encountered in gas-turbine operation over along period of time without cracking of the abrasive layer.

It the abrasive-tipped bucket is called upon to perform its shroud-machining function very often, it may sufier sufficient wear or cracking of the abrasive layer that the bucket will have to be removed and re-tipped, or completely replaced. With the arrangement described above, the non-standard abrasive-tipped bucket is readily removable for such inspection and repair or replacement.

It will be apparent that the invention provides a bucket and shroud arrangement which has no limiting effect on the designers choice of materials for the buckets and shroud segments, which effectively removes interfering material from the shroud segments so as to protect the other standard buckets from interference with the shroud, the single abrasive-tipped bucket being at the same time most likely to require inspection, repair, and replacement, and being the last bucket to be assembled and the first bucket to be removed in the assembly and disassembly process.

The non-standard abrasive-tipped bucket will of course be substantially more expensive than the standard buckets 2, 3, etc., but my arrangement provides excellent over all economy by requiring only one of such nonstandard buckets, and the minimum amount of the expensive abrasive layer material. It will also be obvious that the skill required in applying the abrasive tip portion contributes substantially to the higher cost of the bucket 1 as compared to the standard buckets 2, 3.

Thus, the invention provides optimum economy from the standpoint of manufacturing cost of the rotor, optimum effectiveness from the standpoint of protecting the standard buckets from interference with the stationary shroud ring, and optimum convenience in removing the non-standard bucket for inspection and servicing or replacement. All these advantages are obtained while permitting the absolute minimum radial clearance between the rotating buckets and the stationary shroud ring. These advantages are obtained while using conventional shroud construction employing standard shroud materials. With a segmental shroud as shown in the drawings, it is of course possible to readily replace only those shroud segments which have been worn away by the abrasive bucket, in the event that the replacement of any of the shroud segments becomes necessary.

While the invention has been described as primarily applicable to high-temperature gas-turbine blades, it may also be applicable to the lower temperature blading of axial fiow compressors or of steam turbines, in an axial fiow compressor, the blades may not be of metal but of a plastic or composite glass fiber-resin composition, and the abrasive tip may be of some temperature-resisting resin such as a suitable epoxy resin providing the matrix for the abrasive particles, which may be also aluminum ox de or diamond dust, rather than the tungsten carbide specified herein.

The details of the dovetail bucket attachment described herein may of course be replaced by other equivalent means for fastening the buckets to the wheel rim, one aspect of the invention obviously being not limited to the dovetail configuration described here. be apparent that it is most advantageous that all of the non-standard features be concentrated in one bucket so that all the other buckets may be identical, for optimum manufacturing cost. It is, of course, this consideration which dictates that the abrasive tip be applied to the locking bucket when the dovetail means used to secure the hue ets to the rotor requires such a special locking bucket. The specific dovetail locking arrangement described herein is found to be very advantageous in combination with the single non-standar abrasive-tipped bucket.

Many modifications and other substitutions of equivalents wills be apparent to those skilled in the art, and it is of course intended to cover by the appended claims all such modifications as fall within the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In an axial flow turbomachine having a rotor comprising a body member rotatably disposed within a stationary shroud member and at least one circumferential row of radially extending shroudless blade members each having a base portion secured to the periphery of the body member, the combination of:

a single locking blade member having a base portion mounted on said rotor and constructed and arranged to be assembled last on the rotor after all of the other blade members in said row are installed,

the locking blade member having a tip portion projecting radially beyond the common tip radius of all the other blade members in said row,

said tip portion of the locking blade member having a radially outermost portion comprising a layer of a relatively ductile matrix bonded to the blade tip and having dispersed therethrough abrasive particles with sharp cutting edges in random orientation, whereby interference between rotor and shroud causes the abrasive tip of the locking blade member to remove material from the interfering portion of the stationary shroud, with no interference occurring between the shroud and the tips of the other blades. 2. A shroudless blade member for an axial flow turbo- It will, however,

machine having .a'blade tip portion with a radially outermost abrasive layer composed of:

' a layer of a relatively ductile matrix bonded to the blade tip, and abrasive particles with cutting edges in random orientation dispersed throughout said matrix.

3. A shroudless blade member for an axial flow turbomachine having a blade tip portion with a radially outermost abrasive layer composed of a layer of a relatively ductile matrix bonded to said blade tip, and abrasive particles with cutting edges in random orientation disperse throughout said matrix, said abrasive blade tip portion having a cylindrical outer surface coaxial with the rotor axis, and the upstream and downstream side surfaces of said abrasive blade tip portion being defined by radial surfaces substantially normal to said cylindrical surface.

4. A shroudless blade member for an axial flow turbomachine having a blade portion of a temperature-resistant metallic alloy material with the radially outermost portion thereof composed of a layer of a relatively ductile metallic matrix fused to the blade tip portion and, dispersed throughout said matrix, abrasive particles with cutting edges in random orientation.

5. A turbornachine blade in accordance with claim 4, in which:

the blade-tip portion is composed of an austenitic ferrous alloy, and

' the abrasive tip layer comprises a matrix composed essentially of nickel, having dispersed therein sharpedged particles of tungsten carbide.

6. A shroudless rotor blade for an axial flow turbomachine having a stationary shroud ring surrounding the circumferential row of blades, said blade having:

a radially outermost tip portion composed of a matrix material containing dispersed therein abrasive particles with cutting edges adapted to machine away any interfering portion of the stationary shroud ring.

7. In a shroudless turbomachine rotor assembly having a disk member disposed coaxially within a stationary shroud ring, a plurality of identical bucket members and a single locking bucket member different in construction from said identical buckets, all of said buckets being secured circumferentially around the rim portion of said disk member, said single locking bucket member having a radially outermost tip portion with a layer of abrasive material adapted to remove any interfering portion of said cooperating stationary shroud ring without damage to the tips of said identical buckets, said single locking bucket member with its abrasive tip portion being slightly longer than said identical buckets.

8. A turbomachine bucket in accordance with claim 7 in which said abrasive tip layer comprises a relatively ductible matrix having dispersed therethrough abrasive particles having sharp edges and corners constituting cutting edges for machining away interfering portions of the stationary bucket shroud ring.

References Cited by the Examiner UNITED STATES PATENTS 2,519,557 8/50 Flanagan. 2,843,356 7/58 Hull 253-77 FOREIGN PATENTS 474,361 3/29 Germany. 440,079 12/ 35 Great Britain.

SAMUEL LEVINE, Primary Examiner.

JULIUS E. WEST, Examiner. 

2. A SHROUDLESS BLADE MEMBER FOR AN AXIAL FLOW TURBOMACHINE HAVING A BLADE TIP PORTION WITH A RADIALLY OUTERMOST ABRASIVE LAYER COMPOSED OF: A LAYER OF A RELATIVELY DUCTILE MATRIX BONDED TO THE 